Now, how do you get into the atrium when everything is on fire...anyone, please...?

Go back upstairs into the room with Camille and the (dead) General. Walk around Camille and through the bedroom door. There is a girder that you need to walk across - but get the timing right or the flames will lick up and kill you. Once across, you can go left then down the slope to the atrium.
If you manage it your a better man than me as I have lost count at the number of times I have died.